1. Field of the Invention
The present invention relates to a self-biased solar cell and a module adopting the solar cell, and more particularly, to a self-biased solar cell having improved conversion efficiency, in which recombination loss of carriers at the rear and front of the solar cell; is decreased, and open voltage and quantum yield at long and short wavelengths are increased, and a module adopting the solar cell.
2. Description of the Related Art
A solar cell uses the effect of an optical electromotive force of a semiconductor, and is manufactured by combining a p-type semiconductor and an n-type semiconductor. When light is irradiated onto a junction (p-n junction) between the p-type semiconductor and the n-type semiconductor, negative charges (electrons) and positive charges (positive holes) are generated in the semiconductor by the light energy.
Generally, when light whose energy is lower than a band gap energy is incident on the solar cell, electrons in the semiconductors reacts weakly with each other. Meanwhile, when light having more than the band gap energy is incident on the semiconductor, electrons in covalent bonds are excited, generating carriers (electrons or positive holes).
The carriers generated by the light return to a normal state through a recombination process. The time taken for the carriers to return to the normal state after generation is called "carrier lifetime".
The electrons and positive holes generated by the light energy migrate toward the n-type and p-type semiconductors due to an the internal electric field, to be collected at electrode portions at two ends. When two electrodes are interconnected by a conducting wire, power of desired voltage and current is generated.
Generally, when the electric field is applied to the whole light absorption layer, the lifetime of the carriers excited by the light is increased. Forming the electric field on the light absorption layer can be achieved by controlling a doping profile in a bulk region. However, this method can be applied only when the amount of dopant can be controlled while silicon is grown.
Generally, it is expected that the electric field effect is further increased near the p-n junction rather than at the rear side of the solar cell Also, a back surface field (BSF) is formed by diffusing dopant into the rear side of the electrode. When the electric field is formed at the back surface of the battery, the carriers excited by the light are reflected, thereby reducing loss by the recombination of the carriers and increasing open voltage and quantum effect at long wavelengths.
A buried contact solar cell (BCSC), one of solar cells having the excellent efficiency, has the structure shown in FIG. 1. In order to manufacture a solar cell having such a structure, a back surface electrode 15 is formed by depositing aluminum (Al) and then sintering the deposited Al to form the electric field at the back surface of the battery for reducing contact resistance. Here, reference numeral 11 represents the upper surface of a semiconductor substrate, reference numeral 12 represents a front surface electrode, reference numeral 13 represents groove, and reference numeral 14 represents a semiconductor substrate.
When sintering Al to form the BSF, a thermal process at a high temperature is performed for a long time. Via the thermal process, Al forms a solid solution and the silicon at the back surface of the semiconductor substrate is severely damaged. As a result, carriers are recombined at the back surface of the semiconductor substrate.
To overcome such problems, grooves are formed at the back surface of the semiconductor substrate to form electrode, and phosphate is doped on the remaining portion to form a floating junction film, thereby reducing the recombination of carriers at the back surface. The battery manufactured based on this is a double side buried contact solar cell (DSBC) shown in FIG. 2.
However, such DCBC has a shunt path between a back surface electrode 25 and a floating junction layer 26, so that the effect by the BSF is less than expected.
As described above, the effect of the BSF or the floating junction layer formed in the conventional BCSC and DSBC is less than expected.